The present invention relates to a novel thermosetting powder coating composition and a method for forming a topcoat using the same.
Topcoats for coating substrates such as automotive exterior panels are required to have excellent finished appearance such as surface smoothness and excellent coating properties such as weather resistance, solvent resistance, acid resistance, etc., in view of uses. In recent years, the required levels have been getting even higher.
Thermosetting powder coating compositions are conventionally used as clear coating compositions for forming topcoats by 2-coat 1-bake systems, etc. It is desired that the thermosetting aqueous coating compositions be used as base coating compositions, from the viewpoints of environmental protection and energy saving.
However, the thermosetting powder coating compositions have the following problems. When surface smoothness of the coating film is improved, other properties such as weather resistance, solvent resistance and acid resistance are impaired. On the other hand, when the other properties are improved, surface smoothness is reduced. It has been difficult to achieve a proper balance therebetween. When conventional thermosetting powder clear coating compositions are applied to surfaces of aqueous base coats, such problems develop to a serious level, and it has been difficult to form a topcoat that excels in both surface smoothness and the above-mentioned other coating properties.
For example, a 2-coat 1-bake coating method for forming a topcoat is known from U.S. Pat. No. 5,585,146. This method comprises the steps of applying to a substrate a thermosetting aqueous base coating composition comprising a hydroxyl- and carboxyl-containing resin and an alkyletherified methylol melamine resin, applying to the uncured surface of the base coat a thermosetting powder clear coating composition comprising an epoxy-containing acrylic resin, and a dibasic acid such as dodecanoic diacid or acid anhydride thereof as a curing agent, and curing the two coats by heating.
However, in the above 2-coat 1-bake coating method, when the coat of the aqueous base coating composition and the coat of the powder clear coating composition are heated for curing at the same time, a reaction between the hydroxyl group and the alkylether group in the aqueous coat produces an alcohol as a byproduct. When the alcohol evaporates out through the thermally fused powder coat, it causes cratering due to popping on the surface of powder coat, thereby impairing surface smoothness, distinctness-of-image gloss, and physical properties of the coating film. When the aqueous coat comprises a flaky metallic pigment, orientation of the pigment flakes is altered, thus impairing metallic effects, etc.
In this case, if curing initiation time is delayed so as to fully melt and flow the powder coat particles by heating with the purpose of improving coating surface smoothness, etc., an insufficiently crosslinked powder coat will result, thus impairing coating properties such as weather resistance, solvent resistance and acid resistance.
An object of the present invention is to provide a novel thermosetting powder coating composition that excels in finished appearance such as surface smoothness and also in other coating properties such as weather resistance, solvent resistance and acid resistance.
Another object of the invention is to provide a method for forming a topcoat using the above-mentioned thermosetting powder coating composition.
Other objects and features of the invention will become apparent from the following description.
The present inventors carried out intensive research to overcome the prior art defects and found that the above objects can be achieved with a thermosetting powder coating composition comprising an epoxy-containing resin and a curing agent for the resin containing a specific acid anhydride as an essential ingredient. More specifically, the present inventors found that the above-mentioned powder coating composition is excellent in curability, and when it is used as a clear coating composition in a 2-coat 1-bake coating method using an aqueous base coating composition, a fully crosslinked powder coat will be formed even with the curing initiation time being delayed, thus preventing popping and providing a topcoat with excellent finished appearance such as smoothness and excellent coating properties such as weather resistance, solvent resistance and acid resistance. The present invention has been accomplished based on this novel finding.
The present invention provides a thermosetting powder coating composition comprising:
(A) an epoxy-containing resin; and
(B) a curing agent containing acid anhydride (a) and at least one species selected from the group consisting of acid anhydride (b) and dibasic acid (c),
the acid anhydride (a) being represented by formula
xe2x80x83Xxe2x80x94COOxe2x80x94[COxe2x80x94(CH2)mxe2x80x94COO]nxe2x80x94COxe2x80x94Xxe2x80x83xe2x80x83(1)
wherein the two X""s are the same or different and independently represent a C4-12 alkyl group having a branched structure, R1xe2x80x94Oxe2x80x94COxe2x80x94R2xe2x80x94, R1xe2x80x94COxe2x80x94R2xe2x80x94or R1xe2x80x94COxe2x80x94NHxe2x80x94R2xe2x80x94in which R1 is a C1-10 monovalent saturated hydrocarbon group; R2 is a C1-10 bivalent saturated hydrocarbon group; m is an integer of 6 to 18; and n is an integer of 8 to 50,
the acid anhydride (b) being represented by the formula
HOxe2x80x94[COxe2x80x94(CH2)pxe2x80x94COO]qxe2x80x94Hxe2x80x83xe2x80x83(2)
wherein p is an integer of 6 to 18 and q is an integer of 2 to 18, and having carboxyl groups at both ends,
the dibasic acid (c) being represented by the formula
HOOCxe2x80x94(CH2)txe2x80x94COOHxe2x80x83xe2x80x83(3)
wherein t is an integer of 6 to 18,
the molar ratio of carboxyl groups to acid anhydride groups in the curing agent being in the range between 0.15:1 and 2:1, and
the molar ratio of the total of carboxyl groups and acid anhydride groups in the curing agent (B) to epoxy groups in the epoxy-containing resin (A) being in the range between 0.7:1 and 1.2:1.
The present invention further provides a method for forming a topcoat according to a 2-coat 1-bake system comprising the steps of applying to a substrate a thermosetting aqueous colored base coating composition, applying to the uncured surface of the base coat a clear coating composition, and curing the two coats by heating, wherein the clear coating composition is the above-mentioned thermosetting powder coating composition.
The epoxy-containing resin (A) of the thermosetting powder coating composition according to the invention may be any conventionally known resin that is solid and contains at least one epoxy group, preferably at least two epoxy groups, on average per molecule.
Examples of resins usable as the epoxy-containing resin (A) are bisphenol epoxy resins such as xe2x80x9cEEPIKOTE 1001xe2x80x9d (with an epoxy equivalent of 450 to 500, product of Shell Chemical Co., Ltd., trade name), xe2x80x9cEPIKOTE 1004xe2x80x9d (with an epoxy equivalent of 875 to 975, product of Shell Chemical Co., Ltd., trade name), and xe2x80x9cEPIKOTE 1007xe2x80x9d (with an epoxy equivalent of 1,750 to 2,200, Shell Chemical Co., Ltd., trade name); and epoxy-containing copolymers each prepared by copolymerizing an epoxy-containing vinyl monomer and at least one other polymerizable vinyl monomer.
Examples of epoxy-containing vinyl monomers used in the epoxy-containing copolymers include glycidyl (meth)acrylate, allyl glycidyl ether, 3,4-epoxycyclohexyl (meth)acrylate and xcex2-methylglycidyl (meth)acrylate. The monomers can be used singly or in combination of two or more. Particularly preferred are glycidyl (meth)acrylate and xcex2-methylglycidyl (meth)acrylate.
Examples of other polymerizable vinyl monomers are vinyl aromatic compounds such as styrene, xcex1-methylstyrene, vinyltoluene and xcex1-chlorostyrene; and C1-24 alkyl esters or cycloalkyl esters of acrylic or methacrylic acids, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate and tricyclodecanyl (meth)acrylate.
Of the monomers constituting the epoxy-containing copolymer, the epoxy-containing vinyl monomer is used preferably in a proportion of 20 to 50% by weight, more preferably in a proportion of 25 to 40% by weight. An epoxy-containing vinyl monomer content of less than 20 wt. % in the copolymer reduces acid resistance, weather resistance and solvent resistance of the coating film. On the other hand, an epoxy-containing vinyl monomer content of more than 50 wt. % impairs storage stability of the coating composition and finished appearance such as surface smoothness of the coating film. Therefore, any epoxy-containing vinyl monomer content outside said range is undesirable.
It is preferable for the epoxy-containing resin (A) to have a glass transition temperature of 40 to 80xc2x0 C., particularly 50 to 70xc2x0 C. A glass transition temperature of lower than 40xc2x0 C. causes particles in the powder coating composition to weld together, thereby reducing blocking resistance. On the other hand, a glass transition temperature of higher than 80xc2x0 C. increases the viscosity of the powder coating composition when fused, thus impairing finished appearance of the coating film. Therefore, any glass transition temperature outside said range is undesirable. The glass transition temperature can be measured by DSC (differential scanning calorimeter).
It is preferable for the epoxy-containing resin (A) to have a number average molecular weight of 1,000 to 10,000, particularly 2,000 to 6,000. A number average molecular weight of less than 1,000 reduces blocking resistance of the powder coating composition and impairs coating properties such as solvent resistance and acid resistance. On the other hand, a number average molecular weight of more than 10,000 impairs finished appearances such as surface smoothness of the coating film. Therefore, any epoxy-containing resin having a number average molecular weight outside said range is undesirable.
The curing agent (B) comprises, as an essential curing component, acid anhydride (a) of the formula (1), and further comprises at least one species selected from the group consisting of acid anhydride (b) of the formula (2) having carboxyl groups at both ends and dibasic acid (c) of the formula (3), the molar ratio of carboxyl groups to acid anhydride groups in the curing agent being in the range between 0.15:1 and 2:1.
The molar ratio of carboxyl groups to acid anhydride groups in the curing agent (B) should be in the range between 0.15:1 and 2:1, preferably the range between 0.2:1 and 1.8:1. When the carboxyl group/acid anhydride group molar ratio is lower than 0.15, satisfactory curability is not provided. When the carboxyl group/acid anhydride group molar ratio is higher than 2.0, the curing initiation time occurs earlier, thus causing popping on the coating film and impairing surface smoothness of the coating film. Therefore, any molar ratio outside said range is undesirable.
The molar proportion of acid anhydride groups in acid anhydride (a) to the total of carboxyl groups and acid anhydride groups in the curing agent (B) is preferably 30 to 70 mole %, more preferably 40 to 60 mole %. If the proportion is lower than 30 mole %, the curing initiation time occurs earlier, thus causing popping on the coating film and impairing surface smoothness of the coating film. On the other hand, if the proportion is higher than 70 mole %, satisfactory curability is not provided, thus impairing coating properties such as acid resistance and solvent resistance. Therefore, any molar proportion outside said range is undesirable.
In the formula (1), examples of C4-12 alkyl groups each having a branched structure include 2-ethylhexyl, 3,5,5-trimethylhexyl, neopentyl, neodecanyl or the like. Examples of C1-10 monovalent saturated hydrocarbon groups represented by R1 include a monovalent aliphatic hydrocarbon group, a monovalent alicyclic hydrocarbon group and a combination thereof. Specific examples are methyl, ethyl, n-propyl, n-butyl, n-hexyl, cyclohexyl, n-octyl, n-decanyl and the like. Examples of C1-10 bivalent saturated hydrocarbon groups represented by R2 include a bivalent aliphatic hydrocarbon group, a bivalent alicyclic hydrocarbon group and a combination thereof. Specific examples are methylene, ethylene, n-propylene, n-butylene, n-hexylene, cyclohexylene, n-octamethylene, n-nonamethylene, n-decamethylene and the like.
In the formula (1), m should be an integer of 6 to 18. When m is less than 6, the coating composition is cured too quickly, giving inferior finished appearance. On the other hand, if m is more than 20, the cured coat will have low crosslinking density, thus impairing coating properties such as acid resistance and solvent resistance. Therefore, any integer outside said range is undesirable.
The acid anhydride (a) of the formula (1) can be prepared by conventionally known methods. For example, the compound can be obtained by heating a dibasic acid and at least one monobasic acid selected from the group consisting of a monobasic acid having a branched structure, an ester bond-containing monobasic acid, a ketone bond-containing monobasic acid and an amide-containing monobasic acid, in the presence of acetic anhydride. The compound can also be prepared by a method comprising heating dibasic acid in the presence of acetic anhydride, and monoesterifying part of the acid anhydride groups of the intermediate with an alcohol, followed by further heating the intermediate in the presence of acetic anhydride, thus giving acid anhydride having ester groups at both ends. Examples of alcohols used in the reaction are methanol, ethanol, iso-propanol and the like.
Examples of dibasic acids are aliphatic dibasic acids such as suberic acid, azelaic acid, sebacic acid, undecane diacid, dodecane diacid, tridecane diacid, tetradecane diacid, pentadecane diacid and eicosane diacid.
Examples of monobasic acids as terminal groups are monobasic acids having C4-12 alkyl groups of a branched structure, such as isooctanoic acid, di-n-propionic acid, isononanoic acid and 2-ethylhexanoic acid; monobasic acids having ester groups, such as monopropyl succinate, monobutyl succinate, mono-2-ethylhexyl succinate, mono-n-octyl succinate, monomethyl hexahydrophthalate, monoethyl hexahydrophthalate, monopropyl hexahydrophthalate and monobutyl hexahydrophthalate; monobasic acids having ketone groups, such as levulinic acid; and monobasic acids having amide groups such as acetoamidoundecanoic acid and butylamidoundecanoic acid.
In the formula (1), the group represented by X preferably has a molecular weight of 55 to 230, more preferably 70 to 210. If the group X has a molecular weight of less than 55, unreacted monobacid acid as a starting material distills off under reduced pressure during the synthesis of acid anhydride (a) and it becomes difficult to obtain the desired acid anhydride. Furthermore, when a powder coating composition comprising such a curing agent is baked, a large amount of decomposed monobasic acid volatilizes and accumulates in baking furnaces, filters, etc., thereby damaging the equipment. On the other hand, if the group X has a molecular weight of more than 230, the cured coat of the powder coating composition comprising such an curing agent becomes too soft, resulting in low hardness and has low polarity, resulting in low resistance to solvents such as gasoline and xylene. Therefore, any molecular weight outside said range is undesirable.
The monobasic acid represented by Xxe2x80x94COOH forming the terminal group X in the formula (1) preferably has a boiling point of 200xc2x0 C. or higher, more preferably 220xc2x0 C. or higher. If the boiling point is lower than 200xc2x0 C., unreacted monobasic acid as a starting material distills off under reduced pressure during the synthesis of acid anhydride (a) and it becomes difficult to obtain the desired acid anhydride. Furthermore, when a powder coating composition comprising such a curing agent is baked, a large amount of decomposed monobasic acid volatilizes and accumulates in baking furnaces, filters, etc., thereby damaging the equipment. Therefore, any boiling point lower than 200xc2x0 C. is undesirable.
In the formula (1), n represents degree of polymerization. The polymerization degree can be controlled by changing the feeding ratio of dibasic acid to monobasic acid or the feeding ratio of dibasic acid to alcohol. It is necessary that n be an integer of 8 to 50, preferably an integer of 10 to 45. If n is less than 8, the proportion of monobasic anhydride increases, providing a powder coating composition with low curability. On the other hand, if n is more than 50, the viscosity of the powder coating composition increases when thermally fused, providing a coating film with inferior finished appearance such as low surface smoothness. Hence any degree of polymerization outside said range is undesirable.
Preferably used as acid anhydride (b) of the formula (2) in the curing agent (B) of the invention are acid anhydrides prepared by heating aliphatic dibasic acids such as suberic acid, azelaic acid, sebacic acid, undecane diacid, dodecane diacid, tridecane diacid, tetradecane diacid, pentadecane diacid, eicosane diacid and the like in the presence of acetic anhydride.
In the formula (2), p is an integer of 6 to 18. If p is 5 or less, the curing initiation time occurs earlier, giving inferior finished appearance. On the other hand, if p is 19 or more, the cured coat will have low crosslinking density, impairing coating properties such as acid resistance and solvent resistance. Therefore, any integer outside said range is undesirable. In the formula (2), q represents the degree of polymerization, and is an integer of 2 to 18, preferably an integer of 3 to 15. If q is more than 18, the viscosity of the powder coating composition increases when thermally fused, thus providing a coating film with inferior finished appearance such as low surface smoothness. Hence any degree of polymerization outside said range is undesirable.
Preferably used as dibasic acid (c) of the formula (3) in the curing agent (B) of the invention are aliphatic dibasic acids such as suberic acid, azelaic acid, sebacic acid, undecanoic diacid, dodecanoic diacid, tridecanoic diacid, tetradecanoic diacid, pentadecanoic diacid and eicosane diacid. In the formula (3), r is an integer of 6 to 18. If r is 5 or less, the curing initiation time occurs earlier, giving inferior finished appearance. On the other hand, if r is 19 or more, the cured coat will have low crosslinking density, thus impairing coating properties such as acid resistance and solvent resistance. Therefore, any integer outside said range is undesirable.
The thermosetting powder coating composition of the invention comprises an epoxy-containing resin (A) and a curing agent (B) as essential components. The curing agent (B) essentially comprises acid anhydride (a) in combination with acid anhydride (b) and/or dibasic acid (c).
It is necessary that the molar ratio of the total of carboxyl groups and acid anhydride groups in the curing agent (B) to epoxy groups in the epoxy-containing resin (A) be in the range between 0.7:1 and 1.2:1. If the molar ratio of the total of carboxyl groups and acid anhydride groups in the curing agent (B)/epoxy groups in the resin (A) is lower than 0.7, coating properties such as acid resistance and solvent resistance will be impaired. On the other hand, if the molar ratio is more than 1.2, inferior finished appearance such as low surface smoothness will result. Therefore, any molar ratio outside said range is undesirable.
It is preferable for acid anhydride (a), acid anhydride (b) and dibasic acid (c) in the curing agent (B) of the invention to have melting points of 70xc2x0 C. or higher, more preferably 75xc2x0 C. or higher. If the melting points are lower than 70xc2x0 C., the powder coating composition will have poor blocking resistance, thus being undesirable.
The thermosetting powder coating composition of the invention may contain additives, for example, antipopping agents, surface modifiers, oxidation inhibitors, UV absorbers, UV stabilizers, antiblocking agents, fluidity modifiers, antistatic agents, coloring pigments, metallic pigments, interference pigments, fillers and curing accelerators.
The thermosetting powder coating composition of the invention can be prepared by conventional methods, without limitation. For example, a typical method comprises dryblending an epoxy-containing resin (A), a curing agent (B) and optionally other components in a mixer or the like, and melting and kneading the dryblend with heating, followed by cooling, coarse grinding, fine grinding and sieving.
The thermosetting powder coating composition of the invention may be used as a thermosetting clear powder coating composition or as a thermosetting colored powder coating composition comprising at least one pigment selected from coloring pigments, metallic pigments and interference pigments in the clear powder coating composition. The thermosetting clear powder coating composition may contain such pigments in any amount that the under coat can be seen through or substantially can not be seen through the under coat.
The thermosetting powder coating composition of the invention is applied to a substrate by powder coating and baked to form a cured coating film. It is preferable that the baking be conducted at a temperature of about 120 to about 180xc2x0 C. for about 10 to about 50 minutes.
The substrate to be coated may be any of known substrates that can be coated in a powder coating operation. Examples of useful substrates are metals, surface-treated metals, plastics, or coated these materials.
The powder coating operation can be carried out by conventional methods, for example, preferably electrostatic powder coating methods or frictionally electrified powder coating methods. There is no limitation on the coating film thickness. However, it is preferable for the obtained film to have a coating thickness of about 20 to about 1,000 xcexcm, more preferably about 20 to about 80 xcexcm.
The thermosetting powder coating composition of the invention can be used for vehicles such as automobiles, electric appliances, steel furniture, office equipment and stationery, construction materials and the like where conventional powder coating compositions are used. Particularly, the thermosetting powder coating composition of the invention is suitable for automotive exterior panels or interior panels where high surface smoothness is required of the coating film.
Described below is a method for forming a topcoat on the surface of vehicles or the like, using the thermosetting powder coating composition of the invention.
The method for forming a topcoat using the present invention may be a 1-coat system-or a 2-coat system. The 1-coat system comprises applying the thermosetting powder coating composition of the invention to metallic or plastic substrates for vehicles or the like, or to the metallic or plastic substrates coated with a primer such as a cationic electrocoating composition and optionally coated with an intercoat. The 2-coat system comprises applying a colored base coating composition to the substrates and applying the thermosetting powder clear coating composition of the invention to the coated substrate.
The thermosetting powder coating composition of the present invention produces particularly remarkable effects, when used as a clear coating composition in the method for forming a topcoat according to a 2-coat 1-bake system, which comprises applying a thermosetting aqueous colored base coating composition to a substrate, applying a clear coating composition to the uncured surface of the base coat, and curing the two coats by heating.
Examples of thermosetting aqueous colored base coating compositions preferably used are aqueous colored coating compositions, aqueous metallic coating compositions, aqueous interference pattern coating compositions, aqueous colored metallic coating compositions and aqueous colored interference pattern coating compositions.
According to the 2-coat 1-bake coating method of the invention, a desirable topcoat can be formed by the method comprising the following steps:
i) applying an aqueous colored base coating composition having a solids content of 10 to 60 wt. % (when applied) to the substrate to a coating thickness of about 10 to about 60 xcexcm, preferably about 10 to about 40 xcexcm, when cured, by spray coating such as airless spray coating, air spray coating or electrostatic coating;
ii) allowing the coated substrate to stand at room temperature for about 1 to about 10 minutes or drying the coated substrate at about 50 to about 100xc2x0 C. for about 1 to about 10 minutes;
iii) applying the thermosetting powder clear coating composition according to the invention to the uncured surface of the base coat to a coating thickness of about 20 to about 100 xcexcm, more preferably about 20 to about 80 xcexcm, when cured, by powder coating such as electrostatic powder coating or frictionally electrified powder coating; and
iv) curing the two coats (base coat and clear coat) at the same time by heating at about 120 to about 180xc2x0 C. for about 10 to about 50 minutes.
The aqueous colored base coating composition may be selected from the conventional aqueous colored base coating composition for vehicles, etc. without limitation, of which melamine-curing aqueous colored base coating compositions are particularly preferred.
Preferably, the melamine-curing aqueous colored base coating composition comprises (i) a hydroxyl- and carboxyl-containing resin and (ii) a melamine resin, as curable resin components.
Examples of resins preferably used as the resin (i) include hydroxyl- and carboxyl-containing acrylic resins and polyester resins.
The hydroxyl- and carboxyl-containing acrylic resin can be prepared by copolymerizing a hydroxyl-containing unsaturated monomer, a carboxyl-containing unsaturated monomer, and optionally one or more other vinyl unsaturated monomers. Examples of useful hydroxyl-containing unsaturated monomers are C2-20 hydroxyalkyl esters of acrylic or methacrylic acids, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxybutyl (meth)acrylate. Example of useful carboxyl-containing unsaturated monomers are monocarboxylic acids such as acrylic acid and methacrylic acid; dicarboxylic acids such as maleic acid, itaconic acid, fumaric acid and mesaconic acid; and anhydrides of the dicarboxylic acids or half-esterified or otherwise modified ones. Examples of vinyl unsaturated monomers optionally used are C1-22 alkyl esters of acrylic or methacrylic acids such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate and hexyl (meth)acrylate; styrene, acrylonitrile and the like. It is preferable for the acrylic resin to have a number average molecular weight of about 5,000 to about 100,000, particularly about 15,000 to about 80,000. Preferably, the acrylic resin has a hydroxyl value of about 20 to about 200 mg KOH/g, particularly 40 to 150 mg KOH/g. It is preferable for the acrylic resin to have an acid value of about 10 to about 150 mg KOH/g, particularly 20 to 100 mg KOH/g.
The hydroxyl- and carboxyl-containing polyester resin can be produced by esterification reaction between polybasic acid and polyhydric alcohol. Examples of useful polybasic acids are phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid, HET acid, maleic acid, fumaric acid, itaconic acid, trimellitic acid and anhydrides of these acids. Examples of useful polyhydric alcohols are ethylene glycol, propylene glycol, butylene glycol, hexanediol, diethylene glycol, dipropylene glycol, neopentylglycol, triethyleneglycol, glycerin, trimethylolethane, trimethylolpropane and pentaerythritol. It is preferable for the polyester resin to have a number average molecular weight of about 2,000 to about 100,000, preferably 3,000 to 80,000. Preferably, the polyester resin has a hydroxyl value of about 30 to 120 mg KOH/g, particularly 50 to 80 mg KOH/g. It is preferable for the polyester resin to have an acid value of about 15 to about 100 mg KOH/g, particularly 30 to 50 mg KOH/g.
Resins preferably used as the melamine resin (ii), which is a curing agent, are partially or fully etherified methylol melamine resins prepared by the method described below and having 1 to 5 triazine rings in the molecule and having a molecular weight of about 300 to about 3,000, particularly 500 to 2,000. The method for preparing the partially or fully etherified methylol melamine resin comprises partially or fully methylolating a melamine resin and partially or fully etherifying the methylol groups of the melamine resin using a C1-8 alcohol, such as methanol, ethanol, propanol or butanol. The melamine resin may be hydrophobic or hydrophilic.
The aqueous colored base coating composition may contain, in addition to the components (i) and (ii), at least one pigment selected from the group consisting of coloring pigments, metallic pigments and interference pigments. The aqueous colored base coating composition may further contain oxazoline-containing compounds, extenders, organic solvents, catalysts, sedimentation inhibitors, UV absorbers, etc.
In the case of conventional thermosetting powder coating compositions, when surface smoothness of the coating film is improved, other coating properties such as weather resistance, solvent resistance and acid resistance are impaired. On the other hand, when these coating properties are improved, surface smoothness of the coating film is reduced. Therefore, it has been difficult to achieve a proper balance therebetween. When the conventional thermosetting powder coating compositions are used as clear coating compositions in 2-coat 1-bake systems using aqueous base coating compositions, such problems develop to a serious level. Therefore, it has been difficult to form a topcoat that excels in both surface smoothness and the above-mentioned other coating properties.
In contrast to the conventional compositions, the thermosetting powder coating composition of the present invention comprises, in addition to an epoxy-containing resin, a curing agent containing acid anhydride groups and carboxyl groups in specific proportions. The powder coating composition containing such a specific curing agent is cured after being fully melted and flowed and therefore excels in both surface smoothness and other properties of the coating film, even when used as a clear coating composition in 2-coat 1-bake systems using aqueous base coating compositions, thus achieving remarkable effects.